1. Field of the Invention
The present invention relates to a fundus blood flowmeter for measuring a blood flow velocity on the basis of scattered/reflected light from a fundus portion upon irradiation of the fundus portion with a laser beam.
2. Description of Related Art
Fundus blood flow measurement methods include a method called Laser Doppler Velocimetry (LDV) for measuring an intravascular blood flow velocity and a method called Laser Doppller Flowmetry (LDF) for measuring a blood flow velocity at an optical nerve head (papilla). A technique of allowing one apparatus to execute these two methods is disclosed in, for example, Japanese Patent Application Laid-Open No. 7-136141.
In LDV, measurement light from a measurement light source illuminates a fundus blood vessel in the form of a point, and scattered/reflected light is received by two light-receiving elements such as photomultipliers placed on two light-receiving optical paths defining a predetermined angle. Each light reception signal contains a predetermined beat signal generated by interference between a component Doppler-shifted by blood flowing in a blood vessel Ev and a component reflected by a still blood vessel wall. A blood flow velocity in the blood vessel is obtained by frequency-analyzing this beat signal.
FIG. 10 shows an example of the result obtained by frequency-analyzing a light reception signal obtained by a photomultiplier. Referring to FIG. 10, the abscissa represents a frequency xcex94f, and the ordinate represents, an output xcex94S. The relationship between a maximum frequency xcex94fmax, a frequency vector xcexai, a frequency vector xcexas in the light-receiving direction, and a blood flow velocity vector v is expressed by
xcex94fmax=(xcexasxe2x88x92xcexai)xc2x7vxe2x80x83xe2x80x83(1)
If, therefore, equation (1) is modified by using maximum frequencies xcex94fmax1 and xcex94fmax2 calculated from the light reception signals from the respective photomultipliers, a wavelength xcex of a laser beam, a refractive index n of a measurement region, an angle xcex1 defined by the light-receiving optical paths within the eye, and an angle xcex2 defined by a plane formed by the incidence optical paths in the eye and the blood flow velocity vector, a maximum blood flow velocity Vmax can be given by
Vmax=xcexxc2x7|xcex94fmax1xe2x88x92xcex94fmax2|/(nxc2x7xcex1xc2x7cos xcex2)xe2x80x83xe2x80x83(2)
Such measurements in two directions cancel out the contribution of measurement light in the incidence direction. This makes it possible to measure a blood flow at an arbitrary region on the fundus. In order to measure a true blood flow velocity from the relationship between the nodal line defined by the plane formed by two light-receiving optical paths and the fundus and the angle xcex2 defined by the blood flow velocity vector v and a nodal line A, the nodal line must be matched with the velocity vector v and cos xcex2=1 must be set. For this purpose, the overall light-receiving optical system is rotated or an image rotator is placed in the light-receiving optical system to optically match them with each other.
As an apparatus having a cofocal aperture to eliminate the influence of a blood flow in the choroid during measurement by LDV, the apparatus disclosed in Japanese Patent Application Laid-Open No. 7-79934 is known. Note that an example of measurement of a blood flow velocity in a blood vessel on the fundus is described in Feke, IEEE Transactions of Biomedical Engineering, Vol BME-34, No. 9, September 1987, pp. 673-680 and the like.
In contrast to this, when a blood flow velocity at the papilla is to be measured by LDF, measurement light is applied to the papilla. The light reflected by the papilla is received by a photomultiplier as a light-receiving element. In this case, blood flow velocity vectors, other than the one in a fundus blood vessel in the papilla, have irregular directions, and hence the scattering/reflecting directions of measurement light also become irregular. For this reason, it is said that only one light-receiving direction will suffice. A blood flow velocity at the papilla is obtained by frequency-analyzing this light reception signal.
FIG. 11 shows an example of the result obtained by frequency-analyzing a light reception signal from a blood flow in the papilla. Referring to FIG. 11, the abscissa represents a frequency xcex94f; and the ordinate represents, an output S. The relationship between the frequency xcex94f and the output S can be expressed by an approximate expression like equation (3) given below, and is represented by the thick solid line in FIG. 11.
S(xcex94f)=xe2x88x92Kxc2x7log(xcex94f/xcex3)xe2x80x83xe2x80x83(3)
If equation (3) is modified by using a wavelength xcex of a laser beam, a maximum blood flow velocity Vmax in the papilla can be given by
Vmax=xcex3xc2x7xcex/2xe2x80x83xe2x80x83(4)
As indicated by equation (4), unlike in measurement of a blood flow velocity in a blood vessel, a blood flow velocity in the papilla can be measured regardless of the angle defined by a blood flow and a velocity vector because a measurement light beam is scattered/reflected by a blood flow in the papilla in various directions. Note, however, only a relative velocity is obtained in this case. An example of measurement of a blood flow velocity in the papilla is described in Sebag, et al., Investigative Ophthalmology and Visual Science, Vol 26, No. 10, October 1985, pp. 1415-1422.
In the prior art described above, however, in LDF in which a blood flow velocity in the papilla is measured, a light reception signal received by the photomultiplier contains not only a scattered/reflected component from the papilla but also a regularly reflected component. Consequently, the S/N of an AC component required for frequency analysis is poor.
With ocular movement during measurement, the regularly reflected component from the papilla fluctuates, and the coincidence with approximate expression (4) may become worse in frequency analysis.
It is an object of the present invention to provide a fundus blood flowmeter which solves the above problem and can easily and accurately measure a blood flow velocity at the fundus.
In order to achieve the above object, the present invention is characterized by including an irradiation system which irradiates the fundus with coherent measurement light, a detector which detects scattered light from the fundus upon irradiation of the measurement light, a processor which calculates a blood flow velocity by frequency-analyzing a signal from the detector, and a light shielding aperture which is placed to cover a region irradiated with the measurement light on a plane of the detector which is nearly conjugate to the fundus.
A cofocal aperture which transmits scattered light from the irradiated region and its peripheral portion may be selectively inserted into an optical path in place of the light shielding aperture. In addition, control may be performed such that in the first mode of measuring blood flow information about the papilla, the light shielding aperture may be inserted, whereas in the second mode of measuring a blood flow velocity in a blood vessel on the fundus, the cofocal aperture may be inserted.